Z Spread vs Option Adjusted Spread 223, understand, particular, adjusted

cyber21

Question (Kaplan book):  An investor purchases a bond that is putable at the option of the holder.  The option has value.  He has calculated the Z-spread as 223 basis points.  The options adjusted spread will be:
Answer:: greater than 223 basis points
I dont understand if OAS = z Spread - Option Cost., and the option has value to the investor, why would it be greater (instead of smaller).  
If anyone can further expand on this, and the below LOS in particular, id appreciate it.  I gues I just don’t understand the concept behind it.
For callable bonds, zspread  OAS and option cost  0
Forr putable bonds, z spread